Thanks for the pointers, but none of those links provide concrete
examples. I would like to see a design example for an e-commerce site
or banking app. How would the URLs be encoded? How would a shopping
cart be implemented? A login page? What are the HTTP requests
involved, if you want to do it the REST way?

Fielding uses the term "session state," which is fuzzy. He says "each
request from client to server must contain all of the information
necessary to understand the request, and cannot take advantage of any
stored context on the server." But what does "understanding the
request" mean? What is "stored context"?

Couldn't any data on the server be considered "stored context"? If we
shouldn't have any state on the server, it doesn't even have, say, a
list of books available for purchase. Because that's state. It should
just have a purely functional algorithm, like a mathematical formula. I
don't think that's what Fielding meant, but he doesn't show where to
draw the line.